1. Field of the Invention
The present invention relates generally to tools for well logging, and in particular, to improving the information provided by an electromagnetic (EM) well logging tool.
2. Description of the Related Art
The continuing search for petrochemical materials calls for increasingly aggressive and sophisticated exploration. A necessary part of this exploration involves drilling boreholes into the earth and the coincident creation of deep wells. Examination and evaluation of subterranean formations in the vicinity of the deep wells is completed by a process referred to as well logging.
During well logging, an area near the borehole (i.e., referred to as a near borehole zone) is subject to fluid invasion. Using drilling data and other logging measurements to analyze properties of the invaded zone, one can reach important conclusions about the mineral content.
A variety of tools and techniques for well logging use sophisticated technology for assessing aspects of formations in the vicinity of the borehole. Exemplary well logging tools apply electromagnetic fields, neutron measurements, density determinations and nuclear magnetic resonance (NMR) imaging.
Regarding use of electromagnetic (EM) fields, the resistivity R and other data provided are typically indicative of aspects of surrounding formations that lie in the near borehole zone (considered to be generally from a wall or surface of the borehole and upto about one meter into the surrounding earth) and some distance beyond.
Measurements taken with an electromagnetic (EM) tool can provide valuable information describing a variety of aspects of surrounding formations. However, one skilled in the art will recognize that features of the near borehole zone dominate the results for many measurements. Therefore, it is particularly desirable to produce information describing physical aspects associated with the near borehole zone.
For example, knowing a distribution of the resistivity R, one can make estimations regarding aspects such as filtration capacity for a local reservoir. Furthermore, knowledge and evaluation of the resistivity R within the near borehole zone provides for extracting petrophysical data regarding properties that include permeability, capillary pressure, and relative permeability of the surrounding formations. It is desirable to have knowledge of these properties for the near borehole zone, as each of these properties are useful in making determinations regarding petrophysical conditions.
Although EM tools have considerable value for well logging, typical measurements provide for petrophysical data in areas beyond the near borehole zone. That is, current techniques for use of EM tools do not realize the potential therein for providing meaningful data within (i.e., isolated to) the near borehole zone.
What are needed are techniques for making measurements with an electromagnetic tool, where the measurements provide for determinations regarding petrophysical properties in the near borehole zone.